Gas turbines have a compressor assembly, a combustor assembly, and a turbine assembly. The compressor compresses ambient air, which is then channeled into the combustor, where it is mixed with a fuel. The fuel and compressed air mixture is ignited, creating a working gas that may reach temperatures of 2500 to 2900° F. (1371 to 1593° C.). This gas then passes through the turbine assembly. The turbine assembly has a rotating shaft holding a plurality of circular arrays or “rows” of rotating blades. The turbine assembly also has a plurality of circular arrays of stationary vanes attached to a casing of the turbine. Each row of blades is preceded by a row of vanes to direct the working gas at an optimum angle against the blades. Expansion of the working gas through the turbine assembly results in a transfer of energy from the working gas to the rotating blades, causing rotation of the shaft.
Each vane may have an outer platform connected to a radially outer end of the vane airfoil for attachment to the turbine casing, and an inner platform connected to the inner end of the vane airfoil. The outer platforms for a given row of vanes are mounted adjacent to each other as segments in a circular array, defining an outer shroud ring. The inner platforms are likewise mounted adjacent to each other in a circular array, defining an inner shroud ring. These outer and inner shroud rings define a flow channel between them that channels the working gas over the stationary airfoils.
The vane assemblies may include passages for a cooling fluid such as air. However, the surfaces of the vane assemblies exposed to the working gas are subjected to high operational temperatures and thermal stresses. This can cause cracks in the vane platforms. Typically, each vane airfoil and its two platforms are formed together as a unitary structure, so damage to a platform may require replacement of an entire vane assembly, even when the airfoil is still in a serviceable condition.